As videos, blogs and Web pages created by amateurs remake the entertainment landscape, unknown directors, writers and producers are being catapulted into positions of enormous influence. Each week, about a half-million people download a comedic video podcast featuring a former paralegal. A video by a 30-year-old comedian from Cleveland has now been watched by almost 30 million people, roughly the audience for an average “American Idol” episode. The most popular contributor to the photo site Flickr.com just got a contract to shoot a Toyota ad campaign.